In general, a hydraulic excavator which is a typical example of a construction machine has a vehicle body composed of an automotive lower traveling structure and an upper revolving structure rotatably mounted on the lower traveling structure. A working mechanism for carrying out an excavating work or the like is liftably provided on the front part side of the upper revolving structure.
The upper revolving structure has a revolving frame forming a firm support structural body, the working mechanism is mounted on the front end side of the revolving frame, and a counterweight holding weight balance with the working mechanism is removably mounted on the rear end side. An engine for driving a hydraulic pump is mounted on the revolving frame, as locating on the front side of the counterweight.
Here, a diesel engine is generally used for the engine of the hydraulic excavator. This diesel engine exhausts harmful substances such as particulate matter (PM), nitrogen oxides (NOx) and the like. Thus, in the hydraulic excavator, an exhaust gas post-treatment device for applying purification treatment of the exhaust gas is provided in the middle of an exhaust gas passage of the engine.
This exhaust gas post-treatment device is composed of treatment devices such as a particulate matter trapping filter device (hereinafter referred to as a PM trapping filter device) for trapping and removing the particulate matter (PM) in the exhaust gas, a nitrogen oxide purifying device (hereinafter referred to as an NOx purifying device) for purifying the nitrogen oxides (NOx) by using a urea water solution or a selective reduction catalyst, oxidation catalyst for oxidation removal of carbon monoxide (CO) and hydrocarbon (HC) and the like.
Incidentally, in a hydraulic excavator of a rear small-revolving type suitably used for an excavating work in an urban district and the like, a counterweight is arranged close to the revolving center in order to keep a revolving radius of the upper revolving structure small. Thus, a device accommodating space formed on the front side of the counterweight becomes narrow, and it is difficult to accommodate treatment devices such as the PM trapping filter device, the NOx purifying device and the like constituting the exhaust gas post-treatment device other than devices such as an engine, a heat exchanger and the like in this narrow device accommodating space.
In order to handle this difficulty, a hydraulic excavator in which a recessed portion is provided on the upper surface side of the counterweight is proposed. This hydraulic excavator has the PM trapping filter device fixed to the engine disposed on the front side of the counterweight and the NOx purifying device fixed to the recessed portion of the counterweight (Patent Document 1).
In this hydraulic excavator according to a conventional art, an exhaust pipe of an engine is connected to the PM trapping filter device, and the PM trapping filter device and the NOx purifying device are connected to each other through a pipe. It is so configured that the particulate matter contained in the exhaust gas from the engine is trapped by the PM trapping filter device and the nitrogen oxides in the exhaust gas is purified by the NOx purifying device.